Alpha olefins are commercially produced by the oligomerization of ethylene in the presence of a simple alkyl aluminum catalyst (in the so called “chain growth” process) or alternatively, in the presence of an organometallic nickel catalyst (in the so called Shell Higher Olefins, or “SHOP” process). Both of these processes typically produce a crude oligomer product having a broad distribution of alpha olefins with an even number of carbon atoms (i.e. butene-1, hexene-1, octene-1 etc.). The various alpha olefins in the crude oligomer product are then typically separated in a series of distillation columns. Butene-1 is generally the least valuable of these olefins as it is also produced in large quantities as a by-product in various cracking and refining processes. Hexene-1 and octene-1 often command comparatively high prices because these olefins are in high demand as comonomers for linear low density polyethylene (LLDPE).
Technology for the selective trimerization of ethylene to hexene-1 has been recently put into commercial use in response to the demand for hexene-1. The patent literature discloses catalysts which comprise a chromium source and a pyrrolide ligand as being useful for this process—see, for example, (“USP”) U.S. Pat. No. 5,198,563 (Reagen et al., assigned to Phillips Petroleum).
Another family of highly active trimerization catalysts is disclosed by Wass et al. in WO 02/04119 (now U.S. Pat. Nos. 7,143,633 and 6,800,702). The catalysts disclosed by Wass et al. are formed from a chromium source and a bridged diphosphine ligand and are described in further detail by Carter et al. (Chem. Comm. 2002, p 858-9). The two phosphorous (P) atoms are preferably bridged by an amine (N) bridge and hence these ligands are typically referred to as “P—N—P” ligands. As described in the Chem. Comm. paper, the most preferred P—N—P ligands are those in which each P atom is bonded to two phenyl groups and each phenyl group is substituted with an ortho-methoxy group. Hexene-1 is produced with high activity and high selectivity by these catalysts.
Similar P—N—P ligands are disclosed by Blann et al. in WO04/056478 and WO 04/056479 (now US 2006/0229480 and US 2006/0173226). However, in comparison to the ligands of Wass et al., the disphosphine/tetraphenyl ligands disclosed by Blann et al. generally do not contain polar substituents in ortho positions. The “tetraphenyl” diphosphine ligands claimed in the '480 application must not have Ortho substituents (of any kind) on all four of the phenyl groups and the “tetraphenyl” diphosphine ligands claimed in '226 are characterized by having a polar substituent in a meta or para position. Both of these types of catalysts reduce the amount of hexenes produced and increase the amount of octene (in comparison to the ligands of Wass et al.) and the catalysts are generally referred to as “tetramerization catalysts”.
The performance of Cr bridged diphosphine catalysts is typically temperature dependent. The prior art generally teaches preferred operating temperatures of from 50 to 150° C., especially from 60 to 90° C. Very high activities (of greater than 2×106 grams of product per gram of catalyst per hour) have been reported at this temperature range, particularly when cyclohexane is used as the solvent. However, simple batch experiments have shown that this high activity is also associated with a decrease in product selectivity—in particular, the production of a higher amount of C10+ oligomers has been observed. These C10+ oligomers have comparatively low value so it is desirable to limit the amount of them that is produced.
Batch experiments have shown that product selectivity may be improved by lowering the reaction temperature (albeit, with a lower catalyst activity also being observed).
However, experiments conducted by us under continuous flow conditions showed that a lower oligomerization temperature is not “sufficient” to minimize the C10+ fraction. Instead, a wide range of product selectivity was observed under continuous flow conditions at a given temperature.
We have now discovered that product selectivity can be improved in a continuous process using quite different conditions. More specifically, selectivity can be increased by using a low chromium concentration and by maintaining low octene concentrations in the reactor. Further improvements may be achieved using lower oligomerization temperatures, so low temperatures are preferred (even though a low temperature is not “sufficient” for a continuous process).